1. Field of the Invention
The present invention relates to a sensor for counting pulses of a rotating pulse transmitter for measuring rotational speeds and angular positions of a rotating component to which the pulse transmitter is connected. More specifically, the present invention relates to a magnetically sensitive sensor for counting pulses of a magnetically acting pulse wheel, the sensor being equipped for automatically adapting its switching points to the respective measurement location conditions.
2. Description of the Related Art
In modern motor vehicles, precise determination of the rotational speeds of crankshafts and. camshafts is required. In addition, to reduce fuel consumption and for the purposes of diagnosis, it is also necessary to be able to ascertain the angular position of the crankshaft and camshaft quickly and precisely. For this purpose, magnetically sensitive sensors are arranged so as to be fixed in the immediate vicinity of a pulse wheel such as a gearwheel, magnetic pole wheel or similar magnetically active pulse transmitter which is made of ferroelectric material and fastened on the shaft to be monitored for rotation therewith.
When the pulse wheel comprises a passive pulse wheel such as a ferroelectric gearwheel, the sensor typically comprises a permanent magnet for producing a magnetic field. As the teeth of the gearwheel pass the permanent magnet during rotation, the magnetic field thereof is disturbed. Such a ferromagnetic wheel may comprise an undulatory or acutely delimited tooth structure at the periphery, or may comprise a perforated-plate ring.
If the pulse wheel comprises an active magnetic pole wheel, a multipolar magnetic field rotating at the rotational speed to be measured is produced. Such pole wheels may be provided with a plastic-bonded multipolar ring, a multipolar-magnetised ferromagnetic ring or with individual magnets arranged in annular fashion.
The sensor is fixedly mounted and records the magnetic flux which changes as a result of the rotation of the pulse wheel. The sensor outputs a voltage pulse when a clear signal edge of the magnetic flux occurs. By counting off the pulses occurring per unit of time, the rotational speed may be determined very precisely. A resolution which is essentially defined by the number of teeth on the gearwheel or by the number of poles on the pole wheel may be used to determine the angular position of the relevant shaft as well. To count the pulses, a switching point is used to determine when the pulses occur.
A particular problem of these known sensors is that the signal strength and the gradient of the signal edges decreases rapidly as the distance (clearance) between the sensor and the pulse wheel increases. In this way, installation tolerances or mechanical vibrations have a very pronounced effect. Furthermore, the sensor requires calibration of the switching points to the specific installation.
Other parameters such as, for example, angular offsets between the sensor and the pulse transmitter, tolerances in the flux density of the undisturbed magnetic field, positioning of the permanent magnets, influences of temperature, also affect the magnetic pulses and may detrimentally affect the count result or phase shifts.
Modern magnetically sensitive sensors have a correction mechanism, such as described, for example in Milano, Vig: xe2x80x9cSelf-Calibrating Hall Effect Gear Tooth Sensing Technology for Digital Powertrain Speed and Position Measurementxe2x80x9d, SENSOR 99 Proceedings I, which can be used for dynamically bringing the switching points, which serve to derive a binary output signal, in line with the shape of the edges of the magnetic pulses which occur, so that correct counting is still possible even with shallowing pulse edges and weaker pulses.
For high-precision, time-critical applications, such as determining the ignition instant of a motor vehicle engine, the above-mentioned dynamic correction (self-adaptation) of the switching points of a rotational speed sensor is too sluggish because the correction process takes too long. Furthermore, sensors equipped in this manner run through a start phase, during which their operation is imprecise. With rapid or cyclical changes in the aforementioned parameters caused by vibrations and mechanical jolts which motor vehicles components are regularly subjected to or by the pulse transmitter running eccentrically, the sensors react with an uncontrolled change of their switching points, subject to the dynamic correction, which results in losses of accuracy for the phase angle.
The object of the present invention is to provide a sensor which reliably and precisely measures rotational speed and rotational angle of a rotational component while avoiding the aforementioned problems of the prior art. Furthermore, the sensor should require no manual fine adjustment of the switching points at the fitting location.
The object of the present invention is achieved by the sensor having a logic unit which initiates automatic adaptation of an original state of switching points when the supply voltage of the sensor is turned on and an operating parameter exceeds its initial limit value for the first time. The state of the sensor is not preset or is coarsely preset. After the automatic adaptation, the configuration of the switching points is stored in a non-volatile data memory specifically for the sensor, for constant use.
In a first embodiment of the invention, the prescribed operating parameter is the rotational speed of the rotating component to be detected, and the limit value for said rotational speed is much greater than zero.
In a subsequent embodiment of the invention, the prescribed operating parameter is a temperature, and the limit value for said temperature is significantly above the ambient temperature.
To be able to use the sensor at different fitting locations, and for maintenance purposes, a refined embodiment of the invention provides that the original state can be restored by exerting an external influence.
To this end, provision may be made for the original state to be produced by the logic unit as soon as an output of the sensor is short-circuited to a negative or positive pole of the sensor""s supply voltage (short-circuit monitoring). When the fitting location is changed or in the course of maintenance work, the sensor may thus be easily reset. After the reset, the sensor readjusts itself again when it is next started up. In this context, it is advantageous if the supply voltage is situated outside the nominal operating voltage.
For permanently storing switching points once they have been determined, provision may be made for the non-volatile memory to be an electrically erasable memory, in particular an EEPROM. To reset the sensor, the memory content can be erased in a simple manner which is known per se.
To avoid undesirable influences of temperature on the measurement result, provision may be made for the sensor to have constantly active temperature compensation for the switching points.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.